An out of band (Out Of Band, OOB) refers to an amplitude value in a band in which no data is transmitted and that is out of the range of an operating band in a spectrum. An OOB is an important indicator of a wireless transmission system. A desirable OOB suppression spectrum reduces frequency guard bandwidth used between different communications systems or different services, and improves spectrum efficiency. In addition, a desirable OOB greatly reduces interference between different communications systems (for example, 3G and 4G LTE), thereby improving demodulation performance of a receiver.
Different communications system protocols stipulate a spectrum profile that a waveform applied to a system should satisfy, that is, stipulates a minimum lower limit required by an OOB. As wireless communications technologies are constantly evolved, spectrum resource utilization should be constantly improved. In addition, as application service scenarios are constantly expanded, interfering sources of users from other bands also become increasingly complex. Therefore, a waveform of a low OOB is indispensable for current and future wireless communications systems.
An important factor affecting an OOB is discontinuity of OFDM symbols in a time domain. Because two neighboring OFDM symbols are independent from each other, both an amplitude and a phase are discontinuous. Therefore, energy obtained by adding energies, in a spectrum graph, at remote frequencies located out of a data transmission band is relatively high, that is, an OOB is excessively high, affecting performance of a wireless transmission system.